Gyroscopes have been used to measure rotation rates or changes in angular velocity about an axis. A basic conventional fiber optic gyroscope (FOG) includes a light source, a beam generating device (e.g., a beam-splitter), and a coil of optical fiber coupled to the beam generating device that encircles an area. The beam generating device transmits light beams originating from the light source into the coil of optical fiber, and these light beams propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber. The two counter-propagating (e.g., CW and CCW) beams experience different pathlengths while propagating around a rotating path, and the difference between the two pathlengths produces a phase difference between the two counter-propagating beams that is proportional to the rotational rate.
Many FOGs utilize a glass-based optical fiber to conduct light along a core of the fiber over long distances with low loss and distortion. This optical fiber has a glass/silica core surrounded by a polymer jacket, or buffer, and may be wound into a cylindrical structure, such as a coil, and affixed to a coil-supporting structure, such as a cylindrical hub, to form a sensing coil. The hub and fiber optic coil are both substantially cylindrical structures oriented about a center axis, and the hub has a relatively smaller radius than the radius of the fiber optic coil. An adhesive coating between the outer surface of the hub and inner surface of the fiber optic coil may be used to affix the fiber optic coil to the hub.
Because the optical fiber is a composite structure, the glass/silica core and the polymer buffer may each respond differently to a variety of environmental factors and thereby adversely affect the pathlength difference between the two counter-propagating waves. Some of these environmental factors include temperature and mechanical strain that may create a bias between the phases of the two counter-propagating waves such that the output of the sensing coil yields a phase difference between the two counter-propagating waves that is indistinguishable from a rotation-induced phase difference. During operation, a FOG may be placed in an environment having a fluctuating ambient temperature. Temperature variations affect the sensing coil in two ways: first, the sensing coil undergoes mechanical strain as a result of a differential thermal expansion; and second, the optical transmission properties of the optical fiber change with the temperature. A Coefficient of Thermal Expansion (CTE) mismatch between the glass/silica core and the polymer buffer may result in a transverse expansion of the fiber optic coil that is significantly larger than the lengthwise expansion of the fiber optic coil. Because of the non-isotropic structure of the fiber optic coil, the radial expansion of the fiber optic coil, constrained by the glass/silica core of the optical fiber, is significantly smaller than the axial expansion of the fiber optic coil that is dominated by the large CTE of the polymer buffer. Further, the outer diameter of the fiber optic coil generally expands radially away from the center axis of the fiber optic coil while the inner diameter of the fiber optic coil generally expands radially toward the center axis of the fiber optic coil.
In addition to the expansion of the fiber optic coil, the hub may also expand in response to temperature fluctuations. For example, a hub made from an isotropic material may expand relatively uniformly in both the axial direction and in the radial direction with respect to the center axis. As a result, when a FOG is exposed, to a temperature change such that the fiber optic coil and hub both expand, the hub radially expands faster than the fiber optic coil expands, as a whole, and imparts stress on the fiber optic coil. Additionally, the radial expansion of the hub against the opposite expansion direction of the inner diameter of the fiber optic coil may produce significant mechanical interference between these components resulting in an outward radial pressure exerted at the fiber optic coil interface that induces stresses in the coil structure.
Employing a compliant adhesive, that distorts to accommodate the outward radial expansion of the hub as well as the inward radial expansion of the inner diameter of the fiber optic coil, may minimize such stresses on the fiber optic coil. When the adhesive is softer than the hub material, the stress induced in the fiber optic coil is generally less than the stress induced by the expanding hub alone. The hydrostatic pressure associated with the axial compression of the adhesive material is relieved through its expansion in lateral directions to the extent allowed by hyper-elastic properties of the adhesive material and by the available free area around the adhesive.
One method of applying the compliant adhesive is to inject a liquid adhesive, such as a Room Temperature Vulcanizing (RTV) adhesive, through small orifices in the mounting structure (e.g., a coil hub). Typically, the RTV adhesive is a two-part adhesive that is first mixed and then manually injected through the orifices of the bottom surface of the mounting structure via controlled nozzles. Following a curing period, the two-part adhesive is prepared again and then manually injected through the orifices of the top surface of the mounting structure via the controlled nozzles. The sensing coil mounting process is complete after another curing period. The combination of the curing periods for the RTV adhesive and the manual effort to inject the RTV adhesive generally consumes a significant amount of process time.
In addition to the significant process time, the manufacture of conventional coil hubs is generally labor intensive. Small variations in fiber buffer diameter may accumulate and result in a significant variation in coil height from one fiber optic coil to another fiber optic coil. In general, fiber optic coils are measured after fabrication of the coils, and the coil hubs supporting such fiber optic coils are custom sized to the coils. Typically, the coil hubs are pre-machined and subsequently re-machined after measuring the fiber optic coils. This custom machining of the coil hub complicates the manufacturing process of the FOG. Additionally, when mounting the inner cylindrical surface of the fiber optic coil to the outer cylindrical surface of the coil hub, the clearance between these surfaces may further complicate the mounting process. For example, an RTV adhesive applied to the outer cylindrical surface of the hub may be smeared during the mounting process when the hub and the sensing coil have an insufficient clearance therebetween.
Accordingly, it is desirable to provide a less complex method for attaching a cylindrical inner surface of a sensing coil to a cylindrical outer surface of a support structure in a fiber optic gyroscope while minimizing the coil stress from environmental factors. In addition, it is desirable to provide a sensing coil assembly for a fiber optic gyroscope having minimal construction steps while decreasing the coil stress from environmental factors. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.